The present invention relates generally to an exposure apparatus, and more particularly to a light source apparatus used in an exposure apparatus for exposing an object, such as a single crystal substrate of a semiconductor wafer etc. and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an exposure apparatus that uses an extreme ultraviolet (“EUV”) light as a light source for exposure.
Conventionally, the photolithography technology has employed a reduction projection exposure apparatus using a projection optical system to project a circuit pattern of a mask (reticle) onto a wafer, etc., in manufacturing fine semiconductor devices such as a semiconductor memory and a logic circuit.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution. Thus, along with recent demands for finer semiconductor devices, uses of shorter ultraviolet light wavelengths have been proposed—from an ultra-high pressure mercury lamp (I-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet light has limitations when it comes to satisfying the rapidly promoted fine processing of a semiconductor device. Therefore, a reduction projection optical system using the EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet light (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer very fine circuit patterns of 0.1 μm or less.
The EUV exposure apparatus typically uses a laser plasma light source and a discharge plasma light source as the light source. The laser plasma light source irradiates a laser beam to a target material to generate a plasma and generates the EUV light. The discharge plasma light source generates a plasma by introducing gas to an electrode for discharging and generates the EUV light. The EUV light from the plasma is condensed at a condensing point by a condenser mirror, diverges from the condensing point, and is incident upon a subsequent illumination optical apparatus.
Uniformly illuminating the mask is very important to improve the resolution of the exposure apparatus. Then, a light intensity of the EUV light diverged from the condensing point should be preferably uniform within a divergence angle. However, in the EUV light source operated (used) for a long time, the light intensity of the EUV light that is incident upon the optical system from the condensing point and light intensity distribution within the divergence angle diverged from the condensing point change due to deteriorations of EUV light source's components.
For example, in the laser plasma light source, a nozzle that supplies the target material erodes and deforms by collisions of debris generated from the plasma, and a position of the plasma that generates the EUV light changes. In the discharge plasma light source, the electrode deforms (melts) by the heat from the plasma, and the position of the plasma that generates the EUV light changes. As a result, the light intensity of the EUV light that is incident upon the illumination optical system decreases as a condensing position of the EUV light changes, and the light intensity within the divergence angle diverged from the condensing point displaces from the initial condition.
One proposal detects a generating position of the plasma using a pinhole camera and/or a CCD, controls a target supplying position or a pulsed-laser irradiating position (which is a condensing position of the pulsed-later), and maintains the generating position of the EUV light in place. See, for example, Japanese Patent Applications, Publication Nos. 2000-56099 and 62-283629.
However, the instant inventions have discoursed that the prior art detects only the generating position of the plasma, and it cannot necessarily maintain the light intensity of the EUV light diverged from the condensing point constant within the divergence angle. In other words, the light intensity of the EUV light diverged from the condensing point within the divergence angle depends upon not only by the plasma position but also by other factors, such as a plasma shape, a gas density distribution, and a condenser mirror shape.
For example, the condenser mirror that condenses the EUV light from the plasma on the condensing point lowers its reflectance by the collisions with the debris generated from or near the plasma and by adhesions of contaminations. As a result, the light intensity of the EUV light that is incident upon the illumination optical system may decrease. Moreover, since the condenser mirror does not have a uniformly lowered reflectance occurred on its entire reflective surface, the light intensity distribution within the divergence angle diverged from the condenser point differs from the initial condition. Thus, even with a fixed plasma position, properties of the EUV light fluctuate in the condensing point. Therefore, mere control over the plasma has difficulties in position uniformly illuminating the mask, and deteriorates the performance of the exposure apparatus.
The changes of the properties of the EUV light in the condensing point depend on not only deteriorations of EUV light source's components for a long time use, but also short time or time wise scattering, although a change amount is small. This attributes to temperature changes of the light source, instable supplies of the target material and instable emissions of the laser in the laser plasma light source, and instable supplies of the gas in the discharge plasma light source. These factors similarly deteriorate performance of the exposure apparatus.